Update notes on minSdkVersion

[rkistner]

See discussion in #612.

This now documents SDK 19+ as "supported", but requires additional config.

SDK < 19 is documented as unsupported now - I don't have any test devices for that.

[zx2c4]

Just remove the silly downgrade suggestion, and instead list the proper instructions only for API 19.

[rkistner]

I disagree with that - downgrading the dependency is the simplest, and has the least impact on the build process and the rest of the app. zxing:core 3.3.0 should be perfectly fine for most applications.

It's easy to run into issues with multiDex, which can be tricky to debug if you're not familiar with it (for example forgetting to update the Application class, then getting completely unrelated errors). The instructions are there when you want to go that route.